Process of reducing the oxygen content in gas mixtures

ABSTRACT

A process of reducing the oxygen content in gas mixtures to 0.0-1.5% by volume, in which process the gas is contacted with a solution containing anthrahydroquinone derivatives capable of being oxidized with molecular oxygen under formation of hydrogen peroxide. The supply of oxygen is so adjusted that the amount of oxygen supplied, upon quantitative formation of hydrogen peroxide, stoichiometrically corresponds to not more than 90%, preferably 50%, of the supplied amount of anthrahydroquinone derivative. The hydrogen peroxide content at the contact surfaces between the solution and the gas must not exceed 100 millimols per liter at a simultaneous oxygen gas pressure of not more than 100 millibars. The oxygen-poor gas mixture in accordance with the process of this invention may be used as a protective or inert gas in chemical process industries where use is made of inflammable gases.

The present invention relates to a process for reducing the oxygencontent in gas mixtures that may be utilized as protective or inertgases.

In chemical process industries utilizing inflammable gases and liquids,the requirements for oxygen-poor protective or inert gases areconsiderable. Such is the case for instance in the preparation ofhydrogen peroxide according to the so-called anthraquinone process inwhich anthraquinone derivatives are hydrogenated by means of hydrogengas to anthrahydroquinone derivatives. The anthrahydroquinone derivativeformed is then oxidized with molecular oxygen back to an anthraquinonederivative which forms hydrogen peroxide. One example of such reactionformulae is as follows ##STR1## R=alkyl, for instance C₂ H₅. An inertgas the hydrogenation step of for this process must contain not morethan about 1.5% by volume of oxygen.

The inert gas requirements in the anthraquinone process have hithertobeen satisfied by energy-intensive methods, for instance by closedcombustion of hydrogen with air to water in a nearly stoichiometrichydrogen-oxygen ratio or by combustion of hydrocarbons with air.

In the above-mentioned anthraquinone process for the preparation ofhydrogen peroxide, the oxidation of the anthrahydroquinone derivative iscarried out such that the hydrogen peroxide yield, based upon theanthrahydroquinone content in the starting solution, will be as high aspossible when the oxygen source is air. The oxidation therefore iscarried out with excess air such that the exhaust gas will have anoxygen content of 4-10% by volume. The pressure in the oxidation reactoris usually maintained at 3-6 bars.

The exhaust gas from this oxidation process could be used as aprotective gas for the hydrogenation reaction if its oxygen contentcould be reduced to not more than 1.0-1.5% by volume.

Surprisingly, it has been found that the oxidation cannot simply becarried out with an anthrahydroquinone-oxygen ratio to result in lessthan 1.5% by volume of oxygen in the exhaust gas. When the oxygenpressure is lower than 80-100 millibars, and the hydrogen peroxidecontent in the solution at the same time is higher than 50-100 millimolsper liter, considerable amounts of water are formed, whereby thehydrogen peroxide yield and, thus, the production capacity will bereduced. Under the reaction conditions mentioned above, the reactionrate will be reduced.

On the other hand, if the hydrogen peroxide content of the solution islow, the reaction rate, relatively seen, is rather high, and at oxygenpressures below 80-100 millibars, while at the same time the waterformation is also low. A controlled low content of hydrogen peroxide inthe solution will enhance the possibility of performing the oxidation ata low oxygen pressure, while maintaining a high hydrogen peroxide yield.By thus maintaining the hydrogen peroxide content at a level below about100 millimols per liter, it is possible to reduce the oxygen content ofthe exhaust gas to the desired level of not more than 1.5% by volume.

The present invention thus concerns a process of reducing the oxygencontent in gas mixtures to 0.0-1.5% by volume, in which process the gasis contacted with a solution containing anthrahydroquinone derivativescapable of being oxidized with molecular oxygen to produce hydrogenperoxide, and which process is characterized in that the supply ofoxygen is so adjusted that the amount of oxygen supplied, uponquantitative formation of hydrogen peroxide, stoichiometricallycorresponds to not more than 90% of the supplied amount ofanthrahydroquinone derivative, and in that the hydrogen peroxide contentof the solution at the contact surfaces between the solution and the gasis not more than 100 millimols per liter at a simultaneous oxygenpressure of not more than 100 millibars. The oxygen preferably issupplied in an amount which stoichiometrically corresponds to not morethan 50% of the supplied amount of anthrahydroquinone derivative.

The accompanying FIGURE shows a diagram of an oxidation processaccording to the invention.

The process according to the invention may preferably be carried outcontinuously in any type of oxidation reactor, for instance a tubularreactor or in one or more columns. If a column is used, the solution andthe gas may be conducted in a co-current flow from the top to the baseof the column, or vice versa. Also a counter-current flow may beemployed, i.e. the gas is introduced at the base of the column and thesolution at the top. In order to obtain a large contact surface betweenthe gas and the liquid, the columns preferably are provided withbuilt-in nettings or perforated bottoms, or filled with packings.

The oxidation temperature may amount to 20°-100° C., preferably 40°-70°C. The pressure within the oxidation reactor may be 0.8-10 bars, butpreferably is limited to 1-6 bars. The solvents utilized for theanthrahydroquinone solution may be selected optionally among suchsolvents as are suitable for the anthraquinone process.

The process according to the invention implies a partial oxidation ofthe anthrahydroquinone content in the solution. Thus, this solutioncontains further unoxidized anthrahydroquinone derivative which may becompletely oxidized in any known manner, whereupon the hydrogen peroxidecan be recovered for instance by extraction with water.

The process according to the invention may be utilized to reduce theoxygen content in all types of gas mixtures, provided that they do notcontain any gas capable of reacting with the solvents or the otherconstituents of the solution in an undesired manner.

The exhaust gas which is obtained from the oxidation system usuallyemployed in the preparation of hydrogen peroxide by the anthraquinoneprocess may, like air, be used for generating oxygen-poor gas mixturesaccording to the invention. The product gas can be purified from solventvapours in any known manner, for instance by freezing and/or adsorptionon active carbon. If a very low content of oxygen in the product gas isabsolutely essential, any residual content after the process accordingto the invention has been carried out can be removed, for instance bycatalytic combustion.

Oxygen-poor exhaust gas mixtures from the process according to theinvention may thus be utilized as protective gas in, inter alia, thehydrogenation step of the anthraquinone process. After suitablepurification, they may also be used as protective and inert gases withinother fields of application and may constitute the starting material forthe preparation of, for instance, liquid nitrogen, noble gases andammonia.

To further illustrate the present invention, the following Examples aregiven which are not intended to restrict the invention.

EXAMPLE 1

200 ml of a solution containing 147 millimols oftetrahydroethylanthrahydroquinone (THEAHK) per liter were added to a 500ml flat-bottomed glass flask. The solution as added contained nohydrogen peroxide. The gas phase within the flask consisted of air, andinitially, contained 14 millimols of oxygen per liter of batched THEAHKsolution. The flask was placed in a water bath, the temperature of whichwas maintained constant at 21° C. Disposed within the gas space was anoxygen electrode connected to a recorder which continuously indicatedand recorded the oxygen content of the gas phase. As soon as the neck ofthe flask had been sealed gas-tight, a magnetic stirrer was started inthe solution, and recording of the oxygen content in the gas was begun.After 75 minutes, the oxygen content in the gas had decreased to 0.0% byvolume. The solution then contained 14 millimols of hydrogen peroxideand 133 millimols of THEAHK per liter.

EXAMPLE 2

To illustrate the importance of too high a hydrogen peroxide content inthe solution, the following tests were made.

With the same equipment and at the same temperature, 200 ml of THEAHKsolution were reacted with 0.300 ml of air. The solution initiallycontained 15 millimols of THEAHK and 111 millimols of hydrogen peroxideper liter. The oxygen content of the air was reduced during 200 minutesto 5.3% by volume. After a further 40 minutes, the oxygen content in thegas was still 5.3% by volume. The solution then contained 3 millimols ofTHEAHK and 117 millimols of hydrogen peroxide.

The hydrogen peroxide yield, based upon the amount of oxidized THEAHK,was 50%.

EXAMPLE 3

At the base of a cylindrical column having an internal diameter of 52millimeters and a height of 10 meters and filled with 6 millimetersIntalox saddles of porcelain, 16.5 liters/hour of a solution containing294 millimols of THEAHK per liter were introduced continuously. At thesame time, air was introduced at the column base at a constant flow,such that the oxygen supply amounted to 102 millimols per liter ofsupplied THEAHK solution. Both the exhaust gas and the partiallyoxidized THEAHK solution left the column at the top thereof. Thepressure at the column base was 3.28 bars, and 2.45 bars at the top. Ata continuous steady state, when the temperature in the column was, on anaverage, 51.6° C., the oxygen content of the exhaust gas was 0.6% byvolume. The hydrogen peroxide yield, based upon the amount of oxidizedTHEAHK, was 99%. The oxidation process is shown in the FIGURE.

EXAMPLE 4

11.5 liters/hour of a solution containing 270 millimols of THEAHK perliter were introduced continuously into the same column and in the samemanner as in Example 3. At the same time, a gas mixture containing 5.2%by volume of oxygen and 94.8% by volume of nitrogen was introducedcontinuously. The gas mixture was introduced at a constant flow suchthat the oxygen supply was 313 millimols per liter of supplied THEAHKsolution. The pressure at the column base was 1.92 bars, and 1.18 barsat the top. At a continuous steady state, the temperature in the columnwas, on an average, 50.8° C. and the oxygen content of the exhaust gaswas 3.4% by volume.

The solution leaving the column contained 138 millimols of THEAHK and105 millimols of hydrogen peroxide per liter. The hydrogen peroxideyield, based upon the amount of oxidized THEAHK, was 79.5%.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process of reducingthe oxygen content in a gas mixture to 0.0-1.5% by volume andsimultaneously producing hydrogen peroxide in high yield, comprising,contacting the gas mixture with a solution containing anthrahydroquinonederivatives capable of being oxidized with molecular oxygen to formhydrogen peroxide, the supply of oxygen being controlled such that thesupplied amount of oxygen is at least 10% lower than the amountstoichimetrically corresponding to the supplied amount ofanthrahydroquinone derivative on quantitive hydrogen peroxide formation,and the oxygen supply being so controlled in relation to the supply ofanthrahydroquinone derivative that the oxygen pressure in the gasmixture at the surface of contact between the solution and the gasmixture is not higher than 100 millibars when the hydrogen peroxideconcentration is lower than 100 millimols per liter.
 2. A process asclaimed in claim 1, wherein the oxygen supply is controlled so that theamount of oxygen supplied stoichimetrically corresponds to not more than50% of the supplied amount of anthrahydroquinone derivative.
 3. Aprocess as claimed in claim 1 wherein said contacting of the gas mixturewith said solution is performed at a pressure of 0.8-10 bars.
 4. Aprocess as claimed in any one of the preceding claims, wherein saidcontacting of the gas mixture with said solution is performed at atemperature of 20°-100° C.
 5. A process as claimed in claim 1 whereinthe contacting of said gas mixture with said solution is performed at1-6 bars.
 6. A process as claimed in any one of claims 1, 2 or 3 whereinthe contacting of the gas mixture with said solution is performed at40°-70° C.